Green energy refers to energy produced in such a fashion as to not harm the environment. Examples of green energy sources are photovoltaic panels and wind turbines. It has become customary for utilities in various parts of the world to sell green energy, for which some users are willing to pay a premium. In other cases, utilities or governments use incentives to encourage the purchase of green energy. Some governments encourage the purchase of green energy tokens corresponding to specific quantities of green energy, and in some cases, to specific green energy sources that are effectively assigned to a utility customer.
Green energy production is typically monitored through the use of standard utility-grade meters that are electrically situated between green energy sources and the utility grid. A problem with this setup is the difficulty in verifying the actual energy source. It is relatively straight-forward for someone to replace an expensive green energy source (such as a photovoltaic panel and inverter) with a less expensive polluting energy source (such as a diesel generator).